I have no idea what the problem is. That is an intermittent fault and will be very hard for any service tech to identify. He will have to have his instruments in the right place at the right time to find it. Probably it will require a shot gun replacement of the board that contains the volume control circuit, in the hopes that will cure it.
30 db is a huge increase and puts your speakers at risk.
I'm sorry, but this will probably prove costly with uncertain success.
It is worth doing a receiver reset, if you have not done one already, but I'm not optimistic that will cure your problem.